Many coal deposits contain higher levels of sulfur than are environmentally acceptable, and significant amounts of coal are lost each year in the United States as coal fines and ultra fines because presently used techniques are not very effective in separating such coal from associated shale and sulfur-containing minerals.
According to a U.S. Bureau of Mines study in 1975, the demonstrated coal reserves of the United States are about 430 billion tons, of which only about 255 billion tons are recoverable with present technology. Currently, about 70% of the United States coal is produced from regions east of the Mississippi River; however, most of the eastern coals suffer from a high sulfur content, which on burning emits sulfur dioxide in excess of Environmental Protection Agency limits.
The presence of sulfur in coal is generally attributed to two forms: organic sulfur and pyritic sulfur. Although the proportion of pyritic sulfur to organic sulfur varies significantly from one coal seam to another, it appears that pyritic sulfur generally represents about 70% or more of the total sulfur. The pyritic sulfur is found in coal in a wide size distribution, but a significant proportion is in the very fine size fraction (less than about 25 microns).
For separation of coal from relatively coarse shale and pyrite, gravity-based techniques have been effectively utilized. For example, with sizes below about 300 microns to about 100 microns, froth flotation has been used satisfactorily for separating coal from shale, and even the separation of pyrite from coal has been achieved by flotation. But most of the processes become substantially less effective when the particle size of the coal in mixed ores is significantly below 100 microns.
U.S. Pat. No. 4,211,642, issued July 8, 1980, inventor Petrovich, discloses a froth flotation process for separating pyrite from coal by oil flotation. A relatively low molecular weight (less than 400) polyhydroxy, or aldose compound to which one xanthate group has been added is utilized in the Petrovich flotation process.
Various mixed ore minerals have been separated effectively by the selective flocculation process. For example, hematitic iron ore has been selectively flocculated with a suitable flocculating agent in conjunction with dispersants such as sodium silicate and polyphosphate. Further cleaning of the hemetite flocs is made by either cationic or anionic flotation of silica.
Selective flocculation has also been developed for the recovery of copper minerals from an oxidized copper ore, and such flocculation work has been disclosed by Attia and Kitchener, "Development of Complexing Polymers for the Selective Flocculation of Copper Minerals", which was presented at Proceedings of the 11th International Mineral Processing Congress, Cagliari, Italy, 1975. In this study, it was reported that conventional floth flotation was almost completely ineffective for concentrating the copper minerals from the ore, whereas the use of a very high molecular weight polymer (polyacrylamide modified to include dithiocarbamate groups) effected the selective flocculation separation of copper minerals from associated gangue minerals.
There have been a few attempts to separate coal from shale by selective flocculation. In one attempt, selective flocculation of coal from associated clays was achieved by using a partially hydrolyzed polyacrylamide flocculant and a dispersing agent called "Praestor-211K". Blasche and Sanak, "Separation of Silty Fractions from Coal Slimes by Selective Flocculation", Zesz. Nauk. Alad. Gorn.-Hutn., Cracow, Gorn., 66 (1975) 19-30. In a similar attempt, selective flocculation of coal from shales was achieved by using a non-ionic polyacrylamide flocculant and a sodium hexamethaphosphate dispersant at alkaline pH. However, it has not been known how to remove successfully very fine pyrite particles from coal in a selective flocculation process.